1. Field of the Disclosure
This disclosure relates to the field of electromagnetic well logging. More specifically, the present disclosure is a method of reducing effects of conductive drill pipes on transient electromagnetic signals.
2. Description of the Related Art
Electromagnetic resistivity instruments can be used to determine the electrical conductivity of earth formations surrounding a wellbore. An important objective in Measurement-while-drilling (MWD) applications is the estimation of the distance from the drillbit to an interface in the earth formation that has a resistivity contrast across the interface. The interface may be a bed boundary and/or a fluid interface between a nonconducting fluid (such as a hydrocarbon) and a conductive fluid such as brine.
In a typical transient induction tool, current in the transmitter coil drops from its initial value I0 to 0 at the moment t=0. Subsequently, measurements are taken while the rotating tool is moving along the borehole trajectory. The currents induced in the drilling pipe and in the formation (i.e. eddy currents) begin diffusing from the region close to the transmitter coil in all the directions surrounding the transmitter. These currents induce electromagnetic field components which can be measured by induction coils placed along the conductive pipe. Signal contributions due to the eddy currents in the pipe are considered to be parasitic since the signal due to these currents is much stronger than the signal from the formation. In order to receive a signal which is substantially unaffected by the eddy currents in the pipe, one can measure the signal at the very late stage, at a time in which the signals from the formation dominate parasitic signals due to the pipe. Although the formation signal dominates at the late stage, it is also very small, and reliable measurement can be difficult.
In prior methods, increasing the distance between transmitter and receivers has been used to reduce the influence of the pipe and shift the dominant contribution of the formation to the earlier time range. Besides having limited resolution with respect to an oil/water boundary, such a system requires considerable length (up to 10-15 m). Such large distances are not desirable or convenient for an MWD tool.
There is a need to reduce the parasitic signals cause by eddy currents in transient field signal detection methods without increasing a distance between transmitter and receiver. The present disclosure fulfills that need.